An inverter is an inverting device that electrically converts DC to AC. An inverter used in the industry receives power supplied from a commercial power supply and varies a voltage and frequency of the power and supplies the varied power to a motor. Accordingly, the inverter may control a operation speed of the motor.
A dual high voltage inverter is an inverter whose voltage specification is in a range of 3300 to 11000V. The dual high voltage inverter is used to control a rotation speed of a large capacity motor with a capacity ranging from 600 kVA to 7.5 MVA, or to control a output torque.
In a system that drives a motor using such an inverter, various reasons may lead to restarting the inverter while the motor is rotating. For example, while a commercial power is cut off, such as in a momentary power failure, and thus the motor is free-running, the commercial power is re-input such that the inverter is restarted. This is called a flying start.
In this connection, the inverter has an excessive current, which causes a noise, etc. which is to be suppressed. In severe cases, a power element of the inverter may be damaged. Therefore, a method is required by which the inverter is easily restarted when the power is restored while the motor is in a free running state.
In a conventional case, an output voltage of the motor was measured and analyzed to control the motor based on a change in an active current. However, there is a problem in this approach in that when a residual counter electromotive force exists in the motor, it is difficult to analyze an output current of the motor, which disallows the flying start.
Accordingly, a method for controlling the motor by estimating a frequency of the counter electromotive force of the motor is used. However, in this approach, when a magnitude of the counter electromotive force is small to disallow the frequency estimation, the frequency estimation time becomes longer because a conventional flying start method should be employed.